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Water transfer in the osmotic evaporation of aqueous solutions

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Abstract

Osmotic evaporation (OE) is a new technique [1] allowing to concentrate aqueous solutions at ambient temperature under atmospheric pressure. The water can be extracted from the dilute solution by means of a hypertonic salt solution. A macroporous hydrophobic membrane is used to separate the two circulating solutions. Thanks to its hydrophobicity, the membrane cannot be wetted by the liquids, thus creating vapour-liquid interfaces at the extremities of each pore. The driving force of the mass transfer relies on the vapour pressure difference across the membrane, as a consequence of the water activity difference between brine and dilute solution. So far, few systematic approaches have been adopted to quantify the resistance to mass transfer opposed by the membrane and solutions [2,3]. First, a detailed knowledge of the membrane structure is required to estimate its resistance via gas diffusion models in porous media. Second, the hydrodynamics of the module need to be properly known to characterise the mass transfer resistance in liquid phases. Some authors have studied the contribution of concentration polarisation to the mass flux decay in the bine side, leading to large discrepancies in terms of net flux and polarisation effect. However, concentration polarisation on the other side of the porous wall may also introduce a main limitation, specially when OE is applied to viscous solutions like fruit juices. In this study, a new module was conceived allowing to optimise the circulation conditions of the fluids. Concentration by OE of pure water and diluted sugar solutions with a calcium chloride brine resulted in excellent vapour fluxes, about 10 to 30 times higher than reported in the literature. The effect on flux decay of parameters like sugar and salt content, solution circulation speed and temperature was studied. Among these process variables, concentration effects were by far the most important, acting via activity for the brine and viscosity for the sugar solution. In pure water experiments, the membrane resistance to mass transfer represented 20 to 50 % of the total resistance, and the vapour flux could be doubled by reducing the concentration polarisation in the brine. At elevated sugar contents, the high viscosity of the solutions was the limiting factor. The concentration polarisation in the sugar solutions could not be quantified due to pressure constraints in the equipment that limited the range of speed achievable. These results open promising perspectives for fruit juice concentration by OE.
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Dates and versions

mnhn-03923027 , version 1 (09-01-2023)

Identifiers

  • HAL Id : mnhn-03923027 , version 1

Cite

Mathilde Courel, Manuel Dornier, Gilbert Rios, Max Reynes. Water transfer in the osmotic evaporation of aqueous solutions. Euromembrane '99, Sep 1999, Leuven, Belgium. ⟨mnhn-03923027⟩
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